It sounds like it was recoated with a Teflon product like
this one sold by Brownell's.
Your receiver may and may not be a Supermag. If it is not, do NOT fire 3.5" shotshells,
I tried looked at some code tables. The suffix "A" means it is a SuperMag and the prefix RS means made in November 1998. I would call Remington to confirm this before firing 3.5" shells. I am wondering if your barrel is newer than the gun.